1. Field of the Invention
This invention relates to the field of plaster boards used for building materials.
2. Description of the Prior Art
Plaster boards are used as sheeting for building materials and are covered on one side or on both sides, preferably however on both sides, with paper. These paper layers are firmly combined with the plaster core.
Such plaster boards usually are prepared by spreading on paper, generally having the thickness of cardboard, plaster paste in the amount required for the formation of the plaster board and then preferably covering the upper surface with a second paper (cardboard) layer. This multilayer material is then solidified at elevated temperatures and cut to the desired form before or after the final drying.
Since the paper used for covering the plaster paste generally has the thickness of cardboard, untreated paper can absorb considerable amounts of water from the plaster paste. As a result, the plaster paste within the outer layers becomes drier than that in the interior and crystallizes in a different manner. This phenomenon is termed streaking. The effect can proceed so far that the bond of the paper to the plaster core is weakened and that the layers delaminate. In order to solve this problem, proposals have been made wherein the absorptive capacity of the paper, i.e., cardboard, for water is reduced.
This approach and methods for treating paper with a curable, hydrophobic organosilicone are described in U.S. Pat. No. 3,389,042. This patent discloses the use of several known silicones, such as, for example, epoxyfunctional polysiloxanes, methylhydrogen siloxanes, silanes and siloxanes modified with isocyanate as well as alkoxysilanes and siloxanes prepared from these, as curable, hydrophobic, organosilicone compounds for the treatment of so-called plaster cardboards.
In U.S. Pat. No. 3,431,143, the addition of epoxyfunctional silicones to the paper during its manufacture is described. The paper thus treated is hydrophobized after the curing of the epoxyfunctional silicones.
In U.S. Pat. No. 4,258,102, silicone preparations are disclosed which consist of linear or branched polysiloxanols with a viscosity of 500 to 1,000,000 cP at 25.degree. C. and certain amounts of colloidal silica. These preparations are applied to the paper after its manufacture and are then cured.
It is a common feature of all such known preparations that the organosilicone compounds used could only be cured in an economically short time by using a catalyst. However, the storage stability of the preparations was reduced by the presence of such catalysts. It was moreover necessary to match the paper and the organosilicone preparations in relation to the pH of the paper and the preparations, the salt content of the paper, and especially, the aluminum sulfate content of the paper, so that the organosilicone compounds would neither precipitate nor gel, when the preparations are used.